The primary objective of the present invention is to design golf clubs for a variety of golfers that optimizes the distance the golfer impels the golf ball. To do this from a physics standpoint, it is necessary to obtain a maximum deflection of the ball striking face, or something approaching that maximum, during the collision with the ball while at the same time maintaining the other parameters of the golf club head within acceptable limits.
This spring-like effect of the ball striking face, which is necessary to achieve maximum distance, has been widely misunderstood in the golf industry, even by many golf club designers. Many golf club designers believe that any deflection of the golf club face during impact with its resulting spring-like effect on the golf ball is a design in violation of the Rules of the USGA. This is a myth because virtually all of the thin walled hollow metal wood clubs have significant face deflection during impact and in fact impart a spring-like effect to the ball as it exits the face. This deflection can be as high as in the range of 0.100 to 0.200 inches. And the USGA has approved such clubs although prior to 1999, it did no ball speed or rebound testing on golf clubs. The USGA has now adopted, although in a state of transition, a ball impact club head test in which the rebound speed of the golf ball is measured and compared against the inbound speed of the golf club impacting the club head sample in a stationary position. If the rebound speed of the ball exceeds a certain percentage of the inbound speed, the club will fail the test and the USGA will notify the submitter that the club head has failed the ball speed test and will not be approved by the USGA.
While it is the primary object of the present invention to maximize the face deflection, without causing face failure, and thus maximize face wall energy imparted to the ball, this does not necessarily mean that club heads made in accordance with the present invention will fail the USGA testing, and club heads designed in accordance with the present invention should be submitted to the USGA for such testing and this application makes no representation as to whether such clubs will or will not pass the USGA testing, particularly bearing in mind that the testing procedures and parameters are presently in a state of flux.
In U.S. Pat. No. 4,461,481, issued to Sunyong P. Kim, entitled “Golf Club of the Driver Type”, an internal rod is mounted within the club head extending rearwardly behind the front face and carries a slidable weight 30 that slides back and forth on the rod and impacts the face during ball collision to assist in imparting additional energy to the ball 12. This design is in contravention of the Rules of the USGA because it contains moving parts. It should be noted with respect to the Kim patent, that the present invention contemplates moving parts solely in the sense that the club face deflects and that the USGA has recognized that club face deflection by itself does not constitute a moving part nor is it in contravention of past or present USGA Rules.
In my U.S. Pat. No. 5,873,791, entitled “Oversize Metal Wood with Power Tube”, issued Feb. 23, 1999, and in my following Continuation-In-Part application, U.S. Pat. No. 5,888,148, entitled “Golf Club Head with Power Shaft and Method of Making”, issued Mar. 30, 1999, I describe club head designs in which a power piston is provided to increase the modulus of elasticity of the face wall of the club head throughout the swing speeds in each of the swing speed ranges. The object of the present invention, which is to maximize face deflection, is to reduce the modulus of elasticity in each of the swing speed ranges to achieve maximum face deflection in each of the ranges without causing face failure.
Investment casting techniques innovated in the late 1960s have revolutionized the design, construction and performance of golf club heads up to the present time. Initially only novelty putters and irons were investment cast, and it was only until the early years of the 1980s that investment cast metal woods achieved any degree of commercial success. The initial iron club heads that were investment cast in the very late 1960s and early 1970s innovated the cavity backed club heads made possible by investment casting which enabled the molder and tool designer to form rather severe surface changes in the tooling that were not possible in prior manufacturing techniques for irons which were predominantly at that time forgings. The forging technology was expensive because of the repetition of forging impacts and the necessity for progressive tooling that rendered the forging process considerably more expensive than the investment casting process and that distinction is true today although there have been recent techniques in forging technology to increase the severity of surface contours albeit them at considerable expense.
The investment casting process, sometimes known as the lost wax process, permits the casting of complex shapes found beneficial in golf club technology, because the ceramic material of the mold is formed by dipping a wax master impression repeatedly into a ceramic slurry with drying periods in-between and with a silica coating that permits undercutting and abrupt surface changes almost without limitation since the wax is melted from the interior of the ceramic mold after complete hardening.
This process was adopted in the 1980s to manufacture “wooden” club heads and was found particularly successful because the construction of these heads requires interior undercuts and thin walls because of their stainless steel construction. The metal wood club head, in order to conform to commonly acceptable club head weights on the order of 195 to 210 gms. when constructed of stainless steel, must have extremely thin wall thicknesses on the order of 0.020 to 0.070 inches on the perimeter walls to a maximum of 0.125 inches on the forward wall which is the ball striking surface. This ball striking surface, even utilizing a high strength stainless steel such as 17-4, without reinforcement, must have a thickness of at least 0.125 inches to maintain its structural integrity for the high club head speed player of today who not uncommonly has speeds in the range of 100 to 150 feet per second at ball impact.
Faced with this dilemma of manufacturing a club head of adequate strength while limiting the weight of the club head in a driving metal wood in the range of 195 to 210 gms., designers have found it difficult to increase the perimeter weighting effect of the club head.
In an iron club, perimeter weighting is an easier task because for a given swing weight, iron club heads can be considerably heavier than metal woods because the iron shafts are shorter. So attempts to increase perimeter weighting over the past decade have been more successful in irons than “wooden” club heads. Since the innovation of investment casting in iron technology in the late 1960s, this technique has been utilized to increase the perimeter weighting of the club head or more particularly a redistribution of the weight of the head itself away from the hitting area to the perimeter around the hitting area, usually by providing a perimeter wall extending rearwardly from the face that results in a rear cavity behind the ball striking area. Such a club head configuration has been found over the last two plus decades to enable the average golfer, as well as the professional, to realize a more forgiving hitting area and by that we mean that somewhat off-center hits from the geometric center of the face of the club results in shots substantially the same as those hits on the center of the club. Today it is not uncommon to find a majority of professional golfers playing in any tournament with investment cast perimeter weighted irons confirming the validity of this perimeter weighting technology.
Metal woods by definition are perimeter weighted because in order to achieve the weight limitation of the club head described above with stainless steel materials, it is necessary to construct the walls of the club head very thin which necessarily produces a shell-type construction where the rearwardly extending wall extends from the perimeter of the forward ball striking wall, and this results in an inherently perimeter weighted club, not by design but by a logical requirement.
In the Raymont, U.S. Pat. No. 3,847,399 issued Nov. 12, 1974, assigned to the assignee of the present invention, a system is disclosed for increasing the perimeter weighting effect of a golf club by a pattern of reinforcing elements in the ball striking area that permits the ball striking area to be lighter than normal, enabling the designer to utilize that weight saved on the forward face by adding it to the perimeter wall and thereby enhancing perimeter weighting.
This technique devised by Mr. Raymont was adopted in the late 1980s by many tool designers of investment cast metal woods to increase the strength of the forward face of the metal woods to maintain the requirement for total overall head weight and to redistribute the weight to the relatively thin investment cast perimeter walls permitting these walls to not only have greater structural integrity and provide easier molding and less rejects, but also to enhance the perimeter weighting of these metal woods.
Another problem addressed by the present invention is the achievement of increasing the benefits of perimeter weighting by simply adding weight to the perimeter of the club head itself. This technique, of course, has found considerable success in low impact club heads such as putters, where overall club head weight is in no way critical, and in fact in many low impact clubs that have found considerable commercial success, the club heads weigh many times that of metal wood heads, sometimes three or four times as heavy.
Increased perimeter weighting has been found difficult because of the weight and impact strength requirements in metal woods. An understanding of perimeter weighting must necessarily include a discussion of the parameter radius of gyration. The radius of gyration in a golf club head is defined as the radius from the geometric or ball striking axis of the club along the club face to points of club head mass under consideration. Thus, in effect the radius of gyration is the moment arm or torquing arm for a given mass under consideration about the ball striking point. The total moments acting on the ball during impact is defined as the sum of the individual masses multiplied by their moment arms or “radii of gyration”. And this sum of the moments can be increased then by either increasing the length of the individual moment arms or by increasing the mass or face acting at that moment arm or combinations of the two.
Since it is not practical, except for the techniques discussed in the above Raymont and Allen patents, to add weight to the perimeter wall because of the weight limitations of metal woods and particularly the driving woods, one alternative is to increase the moment arm or radius of gyration. This explains the popularity of today's “jumbo” woods although many of such woods do not have enlarged faces because of the requirement for structural integrity in the front face.
In the Allen, U.S. Pat. No. 5,397,126, an improved metal wood golf club is provided having an enlarged or “jumbo” metal club head with a crowned top wall extending rearwardly from a ball striking face wall, a toe wall, and a heel wall also projecting rearwardly from the face wall—but the club head has no conventional sole plate.
The toe wall and the heel wall are enclosed by the top wall and a pair of spaced generally vertical weighting walls integral with and extending rearwardly from the face wall. The two areas enclosed by the top wall, heel and toe walls, and weight walls are hollow to achieve the desired head weight and the area between the walls is opened, and the weight of the sole plate that normally encloses that area is redistributed to the weight wall to achieve true heel and toe weighting.
Prior attempts to manufacture very large stainless steel metal club heads with larger than normal faces has proved exceedingly difficult because of the 195 to 210 gm. weight requirements for driving club heads to achieve the most desirable club swing weights. Thus, to the present date stainless steel “jumbo” club heads have been manufactured with standard sized face walls, deeply descending top walls from the front to the rear of the club head, and angular faceted sole plates all designed to decrease the gross enclosed volume of the head but which do not detract from the apparent, not actual, volumetric size of the head. This has led to several manufacturers switching from stainless steel to aluminum and titanium alloys, which are of course lighter, to enlarge the head as well as the face.
It has also been suggested in the past that various rods and shafts be cast or attached into the club head for the purpose of rigidifying the forward face wall. However, to the present date, such designs have not achieved any significant commercial success.
The first problem is that, while some of the prior art suggests casting the rods with the forward face, as a practical matter this has never been achieved because of the extreme difficulty in removing the core pieces around the shaft due to interference with the walls of the club head.
A second problem that is not addressed in this prior art is that in order to be effective in reinforcing the front face, the rods need to be integrated into the club head. The rod must also have a weight in the range of 20 to 30 gms. If one simply adds 20 to 30 gram element to a 200 gm. head, the resulting weight of 220 to 230 gms. is excessive and will result in a swing weight far higher than acceptable to the present day average golfer.
An additional problem in many of these prior rigidifying elements is that they are constructed of a low modulus material such as plastic or graphite compositions. These materials do not significantly increase the resonant frequency or the rebound of the face wall. Ideally, the rebound of the face wall; that is, the return of the face wall to its relaxed configuration, should occur at approximately the time the ball exits the face wall. In this way the rebound of the face wall assists in propelling the ball from the club face. If rebound occurs after the ball exits the face wall, the benefits of this effect are completely lost. None of the prior art dealing with these reinforcing elements suggests utilizing this technique for matching face wall rebound with ball exit from the face wall.
A further problem in the prior art references which suggest utilizing these rigidifying elements, is that they are completely silent on how these reinforcing elements, when not cast into the face wall, are attached into the club head. And the method of attachment, as will be seen from the present invention, is critical to the benefits of increasing resonant frequency and rebound of the face wall in accordance with the present invention. Presently known bonding techniques are not sufficient to yield these benefits.
Still another of these prior references suggests making the head of synthetic material and the support rod of a similar material, but these low modulus and soft materials cannot significantly raise the resonant frequency or rebound time of the ball striking face wall.
The following patents or specifications disclose club heads containing face reinforcing elements:
Foreign Patents:                British Patent Specification, No. 398,643, to Squire, issued Sep. 21, 1933;        
United States Patents:                Clark, U.S. Pat. No. 769,939, issued Sep. 13, 2004        Palmer, U.S. Pat. No. 1,167,106, issued Jan. 4, 1916        Barnes, U.S. Pat. No. 1,546,612, issued Jul. 21, 1925        Drevitson, U.S. Pat. No. 1,678,637, issued Jul. 31, 1928        Weiskoff, U.S. Pat. No. 1,907,134, issued May 2, 1933        Schaffer, U.S. Pat. No. 2,460,435, issued Feb. 1, 1949        Chancellor, U.S. Pat. No. 3,589,731, issued Jun. 29, 1971        Glover, U.S. Pat. No. 3,692,306, issued Sep. 19, 1972        Zebelean, U.S. Pat. No. 4,214,754, issued Jul. 29, 1980        Yamada, U.S. Pat. No. 4,535,990, issued Aug. 20, 1985        Chen, et al., U.S. Pat. No. 4,681,321, issued Jul. 21, 1987        Kobayashi, U.S. Pat. No. 4,732,389, issued Mar. 22, 1988        Shearer, U.S. Pat. No. 4,944,515, issued Jul. 31, 1990        Shiotani, et al., U.S. Pat. No. 4,988,104, issued Jan. 29, 1991        Duclos, U.S. Pat. No. 5,176,383, issued Jan. 5, 1993        Atkins, U.S. Pat. No. 5,464,211, issued Nov. 7, 1995        Rigal, et al., U.S. Pat. No. 5,547,427, issued Aug. 20, 1996        
In the Squire British Specification 398,643, the reinforcing rods 10 and 18 are primarily for the purpose of reducing ringing in the face. Squire makes no attempt to maintain head weight within acceptable limits and is completely silent on how the rod 10 can be cast inside the head while removing the core pieces therefrom. Squire is also silent on the rebound or resonant frequency on the head.
The Clark, U.S. Pat. No. 769,939, shows a movable rod that assists in propelling the ball from the club face.
The Palmer, U.S. Pat. No. 1,167,106 shows a weighting element that does not extend completely through the club head.
The Barnes, U.S. Pat. No. 1,546,612, shows rods 13 and 14 extending into the club head, but these rods are for attachment purposes of the face 10 and the club is not a perimeter weighted club.
The Drevitson, U.S. Pat. No. 1,678,637, shows reinforcing partitions 55, but these are not concentrated directly behind the ball striking area, and thus, while rigidifying the face, do not concentrate mass transfer directly to the ball.
The Weiskoff, U.S. Pat. No. 1,907,134, shows a reinforcing member near the center of the club face, but such is not concentrated specifically in the ball striking area and is not a high modulus material.
The Schaffer, U.S. Pat. No. 2,460,435, shows a labyrinth of webs molded in the club head, but the club head is not a high modulus material, nor is the club face and the core 11 is aluminum and not constructed of the same material as the club head.
The Chancellor, U.S. Pat. No. 3,589,731, shows a movable weight between the back and the front of the club that allegedly corrects hooking and slicing.
The Glover, U.S. Pat. No. 3,692,306, shows a weight port integral with the club face in FIG. 6, but Glover's club head is a low modulus resin and is not perimeter weighted.
The Zebelean, U.S. Pat. No. 4,214,754, shows support members 32 in FIG. 10, but they are not connected to the face nor are they concentrated behind the sweet spot.
The Yamada, U.S. Pat. No. 4,535,990, shows a shaft between the rear of the face wall and a back portion of the club, but the Yamada club head is not a high modulus material, and the patent is silent as to how the reinforcement member 31 is connected into the club head cavity.
The Chen, et al., U.S. Pat. No. 4,681,321, shows webs 31 molded inside the club head, but both the club head and the webs are low modulus materials.
The Kobayashi, U.S. Pat. No. 4,732,389, shows a brass plate and a rod that engage the rear of the ball striking face, but the patent is silent as to how it is attached to the face and the club head is solid wood and not a perimeter weighted club head.
The Shearer, U.S. Pat. No. 4,944,515, shows a shaft 24 either cast or attached inside the club head. The Sheer patent is silent as to how the shaft could be cast in the club head and in the alternative suggests that it be fixed in after the club head is made, the patent is silent as to how it might be fixed inside.
The Shiotani, et al., U.S. Pat. No. 4,988,104, shows an insert 15 that is insert molded inside the golf club head, but the club head is a resin type low modulus material, and there is no specific attachment of the insert into the head other than that which results from the insert molding process.
The Duclos, U.S. Pat. No. 5,176,383, discloses a low modulus graphite head having a rod formed on the rear of the ball striking face. The low modulus head provides the Duclos club with minimal perimeter weighting.
The Atkins, U.S. Pat. No. 5,464,211, shows a plate 30 that is threaded from the rear of the club against the forward face which he refers to as a “jack screw”. The plate 30 is epoxied to the rear of the face wall and such a design will fail under the extreme high impact loadings of a 150 ft./sec. impact with a golf ball.
The Rigal, et al., U.S. Pat. No. 5,547,427, shows partitions. In the FIG. 9 embodiment, the rod 74 is placed in tension which detracts from rigidifying the front face. In the FIG. 10 embodiment, the rod 23 is not integral with the front face.
A further principle problem addressed in the present invention has resulted from the use of light-weight alloys to produce “jumbo” or oversized metal woods that are particularly popular in today's golfing market. These use light-weight metals such as high titanium alloys that permit the club head to be made larger, providing increased perimeter weighting and an easier to hit larger sweet spot. However, there is a trade-off to this large sweet spot and that is a diminution in ball distance travel or in short, the ball does not travel as far as it does with smaller stainless steel heads, which concentrate more mass behind the ball. This in part explains why professionals. on the regular tour rarely use very large titanium club heads.
This diminution in ball distance in jumbo titanium alloys, or other light-weight alloy heads, is believed caused by three factors. First, the very large club heads spread the perimeter wall support points from the ball striking area, causing the face to flex more than smaller heads resulting in a badly delayed rebound of the face. If one can imagine a flat horizontal 1″×6″ pine board supported at points two feet apart and a similar board supported at points 10 feet apart, both with a 200 lb. weight in the middle of the boards, the second board will bend substantially more. This oversimplified is what causes in part the greater face flexure in the jumbo metal woods. Secondly, while titanium is a hard material, it has a modulus of elasticity less than half that of ferrous alloys. The lower the modulus, the greater the strain or deflections, for a given load. It should also be noted that today's high titanium alloy jumbo metal wood heads with volumes in the range of 250 to 300 cm.3, have relatively thin wall thicknesses, less than 0.125, and in some cases substantially less than 0.125 inches, which exacerbates the problem of face flexure and slow face rebound.
These three factors all contribute to an incomplete face recovery during ball impact. That is, the club face bends inwardly at ball impact to a state of tension and then returns at some point in time to its normal relaxed position. The rebound of the club face, or its return to its relaxed position, should ideally assist in propelling the ball from the club face. In these prior high titanium jumbo club heads however, the face wall does not fully recover until after the ball leaves the club face, thereby dissipating as waste a portion of the club head energy.
In my application, U.S. Ser. No. 08/859,282, Filed: May 19, 1997, now U.S. Pat. No. 5,873,791, a high modulus golf club head of the “wood” type is provided with a power shaft, a rod for increasing the resonant frequency and decreasing the rebound time of the face, integral at its forward end with the ball striking wall behind the sweet spot and integral with a rear portion of the club head at its rear end. While others have attempted supports for other purposes such as face reinforcement and club sound or feel, they have not been successful because these clubs are either not possible to manufacture, or will fail under the rigors of a 100 to 150 ft./sec. impact velocity against a golf ball.
In that application a jumbo club head in the range of 250 to 300 cm.3 is disclosed constructed of a hard, light-weight alloy such as titanium or beryllium, with an integral power shaft extending from behind the club face sweet spot to a rear portion of the club head.
The power shaft according to that application was constructed of a metal alloy substantially similar to the metal alloy of the club head so it can be welded or fixed integrally to the sweet spot on the rear of the face wall and cast, welded or fixed integrally to a rear portion of the club head at its rear end. While welding similar metals is certainly not a new concept, it is difficult to weld, for example, a 0.625 inch diameter shaft with a 0.035 to 0.049 inch wall thickness directly to the club head face wall and rear wall because the face wall and rear wall, because of their large areas, require higher heating and welding temperatures resulting in heat distortion of the face wall and rear club head.
To obviate this problem, that application discloses a face wall sweet spot and the rear club head portion with cast in annular retainer walls to which the power shaft is welded. These retainers buff the heat sink effect of the face wall and club head portion and minimize heat distortion in these surfaces during welding.
The power shaft according to that invention is a compromise between club head designs to enhance perimeter weighting and increase the sweet spot area, and the ball distance producing designs that concentrate more mass directly behind the ball at impact.
Hence, I disclose in U.S. Ser. No. 08/859,282, a compromise between increased radius of gyration and increased ball distance.
Another important aspect of my U.S. Pat. No. 5,888,148, and my U.S. Pat. No. 5,873,791, is the customizing of the golf club to the swing speed of the golfer. Golfers swing speed differ radically from about 88 ft/sec. up to as much as 180/ft/sec.(123 mph). The club face at impact becomes concave and before or after the ball leaves the face, the face rebounds to its natural shape. The time the ball remains on the face is surprisingly about the same for the slow swings and the fast, but the harder swinger will compress the ball further. Ideally, for both the fast and slow swinger, the face will rebound precisely as the ball is exiting the face to enhance ball exit velocity. But to do this, bearing in mind time of impact, about 5–7 milli/sec., is about the same for all swing speeds, the face must recover at a faster rate for the high speed swing because it has a greater face deflection. To achieve this, the line of woods gives the higher speed swinger a progressively higher face wall resonant frequency than the lower speed swing. Numerous studies have been made analoging the natural or resonant frequencies of bodies to the rebound of the bodies after bending or deformation and those have been adopted here. But it should be noted however, the natural frequency of all linear structures increases with increasing stiffness and decreases with increasing mass.
In a free body system, the natural frequency of the system f is equal to       1          2      ⁢      π        ⁢          ⁢            (              K        M            )              1      /      2      where f is in cycle per unit of time, of a beam pinned at both ends and center loaded, as the face of a golf club, the spring constant K; i.e., force/unit deflection at point of L and is equal to       3    ⁢    EI        L    3  when E is the modulus of elasticity of the material, I is the moment of inertia, and L is the unsupported length.
While titanium is a very hard material, it has a relatively low modulus(E) of 16.8 psi×10·6 compared to stainless steel, which is 30 psi×10·6. And the natural frequency varies as √{square root over (E)} when E is the modulus of elasticity.
Hence, it is when equating the rebound of a titanium face to that of steel the titanium face must be stiffened significantly more and in quantified amounts, and the present invention provides the tools to do that.
As noted above while golfer swing speeds differ greatly, time of ball impact does not and total club head weight stays in the range of 195 to 205 grams for most all swing speeds. Thus to achieve face frequency matching to swing speed, my U.S. Pat. No. 5,873,791, provided a means to vary face stiffness while maintaining about the same overall head weight.
Toward this end the face wall was stiffened in my U.S. Pat. No. 5,873,791, by selecting a power shaft of varying wall thickness, which of course are of different weight, to equate the weights, the rods are provided with transverse weight ports for high density weights, that yield the same overall weight to the club head but varying stiffness and natural frequency to the club face. In this way, faster face rebound is provided for the higher speed golfer and hence slower face rebound for the slower speed golfer to assure that face rebound coincides with ball exit event on the club face.
Using these philosophies, a line of relatively high modulus metal woods was developed, and while stainless steel can be used, the choice is lighter weight alloys having a high surface hardness such as a high titanium or a high beryllium alloy. Utilizing a single club head body tool(the club head bodies are the same initially as are their face walls), the system includes a plurality of interchangeable power shafts providing increasing stiffness and resonant frequency to the ball striking wall, beginning with thin walled shaft for the slower swinger and progressing to a heavy wall shaft for maximum stiffness and higher resonant frequency for the higher swing speed club.
In accordance with my U.S. Pat. No. 5,888,148, a golf club head with a power shaft is provided with an increased modulus of elasticity by preloading the power shaft, and a method of making a golf club head with and without preload is disclosed wherein the club head is cast or formed in forward and rear pieces along a generally vertical parting line, and the two pieces are assembled in clamshell fashion over the power shaft and thereafter the forward and rear pieces are joined by welding or otherwise bonding while the power tube is held in place. In a high volume club head embodiment, above 250 cm.3, constructed of a low modulus alloy compared to stainless steel, the power shaft has a preload, or static compression, to increase the modulus of elasticity of the head and ball striking face. This preloading technique is expanded in another embodiment into a semi-customized line of golf club woods, where the club head modulus of elasticity increases with the golfer's club head speed by progressively increasing preload in the club head line. The power shaft is press fitted into the rear of the ball striking face to reduce bonding and welding difficulties in joining the power shaft to the ball striking face. The modulus of the face wall and the power shaft is enhanced by casting or welding the sole plate of the club head along an axial extent directly to the outer surface of the power shaft thereby increasing its columnar strength. By applying opposite axial clamping forces to the two club head pieces during and after welding or other heat bonding, the power shaft is preloaded into a static compression state. When the forward and rear pieces are joined by welding, the axial force application is maintained for a predetermined time after welding and assures that weld relaxation and wall relaxation will not significantly reduce the power shaft preload.
Toward these ends, the club head assembly, in one embodiment of my U.S. Pat. No. 5,888,148, represents a deviation and improvement from the golf club head disclosed and claimed in U.S. Pat. No. 5,873,791. In that patent, the difficulties in joining the power shaft to the club head have been significantly reduced by a non-invasive joining method. That is, the power shaft is joined to one or both of the club head forward and rear pieces without requiring entry into the club head cavity with a welding tool or other joining instrument. This is accomplished by the provision of a tapered socket and cooperating tapered projection on the power shaft that when forced together under high pressure, the press-fitted tapers create a joint far superior to other bonding techniques, such as epoxy, and one that eliminates heat distortion and other problems associated with the welding of the power shaft.
The power shaft may be cast with one of the forward and rear pieces, but preferably it is initially formed separately therefrom. As a manufacturing expedient, it is preferred to form the power shaft as a separate molding or forging because it is difficult to control the power shaft dimensional integrity when cast integrally with either the forward or rear piece.
The sole plate has a concave spheroidal central portion that extends upwardly toward the power shaft. The sole plate has edges that are welded or integrally cast with axial portions of the sides of the power shaft. This design significantly increases the columnar modulus of elasticity of the power shaft without increasing weight because it uses the sole plate as a support, and in effect the power shaft forms a part of the sole plate to further increase the strength of the sole plate itself. This is also a significant weight saving technique. Firstly, because the power shaft forms part of the sole plate, sole plate weight is reduced, and secondly, the power shaft modulus is increased without any increase in weight in the power shaft.
Another aspect of my U.S. Pat. No. 5,888,148, is the incorporation of the power shaft preloading technique into an entire line of “wood” type club heads. In this embodiment, variable modulus of elasticity of the club head face wall is achieved, not by providing variable power shaft wall thickness, as in my application, U.S. Ser. No. 859,282, but rather by varying the magnitude of the static preload of the power shaft acting on the rear face of the club head ball striking wall. Preload variation is carried through a semi-customized line of drivers(or fairway woods) including, for example, four differently preloaded drivers. The first driver is designed for the very low swing speed golfer, the fourth for the highest swing speed golfer. With this technique, the first driver has a power shaft preload of about 20 kg., and the fourth has a preload of about 100 kg. The second and third drivers in the line have proportionately intermediate preloads for the intermediate swing speeds.
In short, a high swing speed golfer plays with the highest preload club head, and the lower swing speed golfer plays with a progressively lower preloads depending upon their individual swing speeds.
In my parent application, U.S. Ser. No. 09/344,172, Filed: Jun. 24, 1999, I disclose a piston that is spaced from the rear of the face wall that impacts the face wall near its maximum deflection point.
It is a primary object of the present invention to reduce face modulus to provide maximum face flexure.